Planning calendars in Europe and much of Asia, factory schedules, retail reporting and many software systems refer to weeks by number: “deliver in week 41”, or 2026-W41. This calculator gives the ISO 8601 week number for any date, the Monday and Sunday that bound that week, the common US-style week number for comparison, and a table of every week in the year.
How to use the week number calculator
- Enter a date. It defaults to today.
- Read the ISO week number on the tape, together with the ISO year it belongs to. Near New Year these two can differ from the calendar year.
- Use the ISO weeks table below the calculator to find the start and end dates of any other week in the same year.
ISO 8601 week rules
The international standard ISO 8601 defines week numbering with three rules:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year’s first Thursday (equivalently, the week with January 4 in it).
- A year has 52 or 53 weeks; days at the very start or end of a calendar year can belong to the neighboring week-numbering year.
A convenient way to compute the week number is to jump to the Thursday of the same week, because the Thursday always lies in the correct week-numbering year:
The ISO week date writes a day as YYYY-Www-d, with d = 1 for Monday through 7 for Sunday. Sunday, October 11, 2026 is 2026-W41-7.
Worked examples
Sunday, October 11, 2026. It is ISO weekday 7, so its Thursday is three days earlier: October 8, day 281 of 2026. ⌊(281 − 1) ÷ 7⌋ + 1 = week 41, running Monday, October 5 to Sunday, October 11.
Monday, December 29, 2025. Its Thursday is January 1, 2026, day 1 of 2026, so the date is in week 1 of 2026.
Friday, January 1, 2027. Its Thursday is December 31, 2026, day 365, which gives ⌊364 ÷ 7⌋ + 1 = week 53 of 2026. 2026 starts on a Thursday, so it is a 53-week year.
ISO weeks vs. US weeks
| ISO 8601 | Common US convention | |
|---|---|---|
| First day of the week | Monday | Sunday |
| Week 1 | contains the first Thursday | contains January 1 |
| Weeks per year | 52 or 53 full weeks | 53 or 54, first and last partial |
| Spreadsheet function | ISOWEEKNUM | WEEKNUM (default) |
| Used by | Europe, much of Asia, many ERP systems | US payroll calendars, many US planners |
For October 11, 2026 the two systems give different answers: ISO week 41 but US week 42, because the US week starting Sunday, October 11 is already the next one.
Years with 53 ISO weeks
A 53rd week happens about 71 times every 400 years, roughly every five or six years. It occurs when the year begins on a Thursday, or in a leap year beginning on a Wednesday:
| Year | January 1 | ISO weeks |
|---|---|---|
| 2020 | Wednesday (leap year) | 53 |
| 2025 | Wednesday | 52 |
| 2026 | Thursday | 53 |
| 2032 | Thursday (leap year) | 53 |
| 2037 | Thursday | 53 |
Businesses that report by week, including retailers using a 52/53-week fiscal year, need to plan for that extra week so year-over-year comparisons line up. To find the weekday for any date, use the day of the week calculator; to step forward a set number of weeks, use the date calculator.
Frequently asked questions
What is the ISO week number?
ISO 8601 numbers weeks from Monday to Sunday. Week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4. Every week belongs entirely to one week-numbering year.
Why is December 29, 2025 in week 1 of 2026?
That Monday starts the week that contains Thursday, January 1, 2026. Because the Thursday is in 2026, the whole Monday-to-Sunday week counts as week 1 of 2026, including the three December days.
Which years have 53 ISO weeks?
A year has 53 weeks when January 1 is a Thursday, or when it is a leap year starting on a Wednesday. Recent and upcoming examples are 2015, 2020, 2026, 2032, 2037 and 2043.
How is the US week number different?
The common US convention starts weeks on Sunday and makes the week containing January 1 week 1, even if it has only one day. That is what spreadsheet WEEKNUM functions return by default, so it can differ from the ISO number by one.